The invention relates generally to electrochemical machining. More particularly, the invention relates to an electrode and an electrochemical machining process for forming non-circular holes.
The efficiency of a gas turbine engine is directly proportional to the temperature of turbine gases channeled from the combustor of the engine and flowing over the turbine blades. For example, for gas turbine engines having relatively large blades, turbine gas temperatures approaching 1500° C. (2,700° F.) are typical. To withstand such high temperatures, these large blades are manufactured from advanced materials and typically include state-of-the-art type cooling features.
A turbine blade is typically cooled using a coolant such as compressor discharge air. The blade typically includes a cooling hole through which the air passes. The cooling holes commonly have an aspect ratio, or depth to diameter ratio, of more than 300:1, with a diameter as small as a few millimeters.
A specialized adaptation of electrochemical machining, known as shaped-tube electrochemical machining (STEM), is used for drilling small, deep holes in electrically conductive materials. STEM is a non-contact electrochemical drilling process that can produce holes with aspect ratios more than 300:1. It is the only known method that is capable of manufacturing the small, deep holes used for cooling blades of efficient gas turbines.
In the STEM process, an electrically conductive workpiece is situated in a fixed position relative to a movable manifold. The manifold supports a plurality of drilling tubes, each of which are utilized to form an aperture in the workpiece. The drilling tubes function as cathodes in the electrochemical machining process, while the workpiece acts as the anode. As the workpiece is flooded with an electrolyte solution from the drilling tubes, material is deplated from the workpiece in the vicinity of the leading edge of the drilling tubes to form holes.
Commonly assigned U.S. patent application Ser. No. 12/118,791, which was published as US 2009/0277803, Bin Wei et al., “Method and Tool for Forming non-circular holes using a selectively coated electrode,” which is herein incorporated by reference in its entirety, discloses a method for forming a non-circular hole from a substantially circular hole. However, this process requires two separate steps. First, a substantially circular hole is drilled with a traditional STEM cathode, and then another cathode is inserted into the circular hole to form the non-circular hole.
It would therefore be desirable to have a single-step STEM process to form non-circular holes. It would further be desirable to have a single cathode that can be used to perform the entire STEM process for forming the non-circular holes.